A multileaf collimator is used in radiation therapy for treating tumors. A multileaf collimator of such kind is described in, for instance, DE 196 39 861 A1 and WO 00/46813. A tumor is irradiated during radiation therapy with energy-rich beams, usually high-energy X-radiation from a linear accelerator. The multileaf collimator is therein brought into the path of the X-ray beam. The multileaf collimator has a plurality of leaves that can be mutually displaced under motorized control for the purpose of establishing an opening whose contour corresponds to that of the tumor. Thus, only the tumor and not adjacent healthy body tissue will be irradiated with the X-rays. Two sets of leaves are for that purpose arranged mutually opposite such that they can be moved with their front sides toward or away from each other. Virtually any tumor contour can be reproduced in that way.
Said leaves can each be positioned by means of an electric motor embodied as a stepping motor. The positioning accuracy of a stepping motor of said type has, though, proved disadvantageous. Moreover, a stepping motor has a starting behavior that does not allow slight adjustment in positioning.